A phaser, also termed a phase change mechanism, is a device used in engines to vary dynamically the instant, or phase angle, in the engine cycle when the intake and/or exhaust valves of the engine open and close. Such devices are known which are incorporated in the drive pulley of the camshaft and which comprise a drive member connected to the camshaft and a driven member connected to the crankshaft. The drive member normally rotates with and at the same speed as the driven member but when it is desired to change the phase of the camshaft, the two members are rotated relative to one another.
In a vane-type phaser, as described for example in EP 0799976, EP 0807747 and GB 2369175, the rotation of the drive member relative to the driven member is effected hydraulically. A vane movable with one of the two members is received in an arcuate cavity of the other member and divides the cavity into two variable volume working chambers. When a hydraulic pressure medium, usually engine oil, is prevented from entering into or being discharged from the working chambers, the drive and driven members rotate as one. On the other hand, when pressure medium is pumped into one chamber and discharged from the other, the members are rotated relative to one another to change the phase of the camshaft relative to the crankshaft.